Groundwater and surface water contamination by toxic organic pollutants has become an increasingly important environmental problem. The contaminants often consist of low molecular weight organic compounds that are hydrophobic and/or volatile. Such contaminants are of concern both in terms of water and air pollution. These compounds are often found in trace concentrations and are not removed by conventional water treatment systems.
One prior art method of reducing such contaminants is air stripping. In this method, the contaminant is removed from water by forcing air through the water. Volatile contaminants are transferred from the aqueous phase to the air. While this process purifies the water, it pollutes the air, since the process does little to transform the compounds to a harmless form.
Some prior art systems for removing hydrophobic contaminants are based on activated carbon. Activated carbon is highly porous and is capable of adsorbing many contaminants from both liquid and gas phases. The contaminated gas or liquid is typically passed through an activated carbon bed. The contaminants are then adsorbed by the activated carbon until the capacity of the bed is reached. At this point, the activated carbon must be regenerated. The regeneration process typically involves heating the carbon to extremely high temperatures at high pressures. This process is expensive and results in significant carbon loss. In addition, the adsorptive capacity of activated carbon is low for many contaminants.
An alternative method for regenerating the activated carbon involves biological regeneration. In this method microorganisms consume the contaminants trapped on the activated carbon. Unfortunately, the contaminants are often released too slowly from the activated carbon for such biological regeneration to be practical.
Another problem inherent in systems based on activated carbon is the lack of selectivity. Activated carbon sorbs a wide range of organic compounds. The various contaminants compete for the available active adsorption sites. Generally, it would be advantageous to remove only the low molecular weight compounds of concern by sorption on the activated carbon, thereby effectively increasing the capacity of the activated carbon system. Unfortunately, the heterogeneous nature of activated carbon makes such selective sorption impossible. As a result, more activated carbon is required to remove the low molecular weight contaminants of interest than would be the case if a more selective material were available.
Decontamination systems utilizing microbiological degradation of the contaminants are also known to the prior art. Unfortunately, the concentration of contaminants in the waste stream is seldom at the optimal concentration for the microorganisms to achieve rapid transformation of the contaminants. The efficiency of such systems is often very dependent on the concentration of the substrate being transformed. At low levels of contamination, large volumes of water or gas must be brought into contact with the microorganisms for significant periods of time. The cost of the reaction vessels for accomplishing this is high. In addition, the mixing processes needed to achieve rapid biotransformation can result in the release of the organic material into the atmosphere. Finally, biofouling of the effluent often occurs. Such fouling results in a high maintenance cost. At high concentrations, the contaminants to be transformed or other contaminants present in the water or gas may become toxic to the microorganisms.
In addition, some contaminants cannot be used as primary substrates for energy and growth by the microorganisms but can be transformed or destroyed through cometabolism. Cometabolism is the fortuitous transformation of one substrate by an organism that is utilizing a substrate for energy or growth. Providing the substrate in the waste stream is difficult in many cases.
Broadly it is an object of the present invention to provide an improved decontamination method for removing small molecular weight organic compounds.
It is a further object of the present invention to provide a decontamination method which does not require pryrolysis of activated carbon.
It is yet another object of the present invention to provide a microbial decontamination process which is less sensitive to the concentration of the contaminants in the water or gas stream to be decontaminated.
It is another object of the present invention to provide a microbial decontamination process which is less sensitive to the presence of large molecular weight organic compounds in the waste stream being decontaminated.
It is a still further object of the present invention to provide a microbial decontamination system which does not require large reaction vessels for holding the contaminated water or gas in contact with the microorganisms.
It yet another object of the present invention to provide a microbial contamination system which has reduced biofouling.
It is a still further object of the present invention to provide a microbial decontamination system in which the degradation of contaminants via cometabolism may be enhanced.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the present invention and the accompanying drawings.